A conventional dry grind milling process is shown in FIG. 1. As shown, the modern ethanol dry milled facility begins with a feedstock at step 10, such as field corn or sorghum, grinds that feedstock into flour with a hammer mill 12 then wets that flour to create a mash 14. Customarily, the mash is heated to about 180° F. and is cooled to about 90° F. prior to fermentation. The mash is customarily at a pH of between about 4.0 to 5.5. The mash 14 may be further ground in a fine grinding operation 16 or it may be transferred directly to a liquefaction tank 18 where enzymes are added to convert the starches in the mash to sugars 18. These sugars are thereafter converted into ethanol through fermentation 22. Following fermentation 22, the entire mix from the fermentation vessel is delivered to a distillation process 24 where it is further heated with the alcohol removed from the mix by vaporization which is recaptured by condensation thereby providing the primary product, ethanol 28, 30. Other purification steps, such as the use of molecular sieves 26, may also be employed in the process. The process mix remaining after the distillation step 24 is generally referred to as whole stillage.
The focus of the ethanol industry has traditionally been on the output of alcohol and maximization of production from the available starch for conversion. This conversion was accomplished in the front end of an ethanol plant, and once the alcohol was removed from the process stream, the plant was left with the task of disposing of large volumes of whole stillage. Traditionally, the “front end” of the process refers to the process steps which occur before fermentation and the “back end” refers to the process steps which occur after fermentation. In the early days of the modern ethanol production industry, this problem was simply addressed by removing water from the whole stillage and selling the remainder as a by-product for use as animal feed. This by-product was called distiller's dried grain (DDG) or distiller's dried grain with solubles (DDGS).
Beginning about the early 2000s, the industry began to also focus on the potential products presented by the whole stillage. As a result of sheer volume and water content, the industry focus was and remains on dewatering of whole stillage which, in the past, presented the opportunity to sell DDG or DDGS and, more recently, presents the opportunity to fraction the contents of the whole stillage to recover more useful by-products.
The dewatering process of the whole stillage follows distillation. The whole stillage is sent to a separation step 32. At the separation step 32, often involving the use of large horizontal centrifuges, the whole stillage is separated into a primarily heavy solids stream referred to as wet cake. The wet cake was historically sent to driers 36 for further dewatering in the production of DDGS.
The second stream emerging from the separation step 32 is a primarily liquid stream which contained mostly water with dissolved solubles and, also, a small percentage of solids. This liquid stream was referred to as the thin stillage steam. A portion of the liquid/thin stillage is recirculated back to the slurry mix 14 (stream labeled “back set” in the figures) so as to reduce the amount of fresh water introduced into the process and, further, to reduce the amount of thin stillage that required processing via the evaporators. Historically, the thin stillage is sent to the evaporation step 34 where it was subjected to a series of dewatering processes by circulation through multiple evaporators 34. One of the historic ethanol plant designs used a series of eight evaporators through which the thin stillage stream flowed, usually in series. In early ethanol plant designs, after completing the evaporation step 34, the resulting concentrated thin stillage stream was generally referred to as syrup, and the syrup was customarily added to the wet cake content in the drier 36 where further dewatering occurred in the production and recovery of DDGS 38. Optionally, if a nearby market existed, the syrup could be sold separately as a liquid animal feed supplement and, as it was not added to the wet cake in the drier 36, the resulting solid feed product that emerged from the driers was DDG. Also, with nearby markets, the wet cake could bypass the drier operation and be sold as wet DDG or the syrup could be sprayed upon the wet cake, with this mix bypassing the driers and sold as wet DDGS for use as animal feed.
In the 1950s, it was understood that the content of oil available from corn harvested in the United States was highly variable. In the article by Lofland, et. al., Distribution of Fatty Acids in Corn Oil, The Journal of the American Chemists Society, Vol. 31, 412-413 (October 1954), oil content in corn was found to vary from 1.13% to 13.80%. Also, when ethanol plants began operations in earnest, there was no ready market for corn oil and, further, there were no specifications utilized by plants that mandated the corn feedstock material contain a particular percentage of corn oil. This was in line with the industry focus on alcohol output and efficiencies of fermentation (i.e.—implementation of fine grind operations on the mash to expose more surface area of starch in the resultant mash which would undergo fermentation). As the focus in the industry was beginning to expand to include matters other than the efficiency and output of alcohol, the infant biodiesel industry began to move away from the use of soybean oil as its feedstock replacing it with corn oil. This emerging market propelled the ethanol industry to add well known processes of oil separation using centrifuges to their backside operations which processes were used to separate oil from the thin stillage stream. This oil separation process was most often implemented near the evaporators as it afforded the plant the opportunity to control the water content of the in-feed stream to the centrifuge so as to facilitate the mechanical separation of the corn oil. In operation, the thin stillage stream was frequently diverted from the piping connecting two of the evaporators; it was sent to centrifuges where it was processed with oil separating from the stream, and the thin stillage stream was thereafter returned to the piping system so as to flow into the evaporation unit where it was headed prior to diversion. As noted above, this stream would frequently thereafter be sent to the drier to become a part of the DDGS. The oil portion separated from the thin stillage stream was often further processed by storage in vessels that allowed the solids captured by centrifugation to separate with the remainder sold as corn oil.
One problem with traditional processes is that the thin stillage includes emulsions which capture and bind oil. The bound oil is difficult to separate from the thin stillage. Therefore, there is a need for a process which increases the amount of oil recovered from ethanol plants.